Sandblast and IPS look for different types of threats and it is recommended you deploy both.
IPS is looking at network traffic in general, preventing threats that can occur due to malicious use of known flaws.
For example, there are attacks specifically against the SMB protocol that made the news recently.
With updated signatures and Security Gateways in the proper locations, those sorts of attacks can be prevented.
This is, of course, just one of thousands of examples.
SandBlast is looking at Office and PDF files to see if they are malicious through emulation.
This is not something IPS is designed to handle.
Likewise, Sandblast isn't looking at things like the SMB protocol.